The first planet to be discovered around a regular star was made in 1995 and orbits its host star 51 Peg every 4.2 days. In the nearly 30 years since the first discovery of an exoplanet (as planets orbiting stars other than our Sun are called) we now know of 5450 confirmed planets. Many of these initial detections were made using the `transit’ method where a planet can cause a dip in the host stars brightness.

If an exoplanet has an orbit around its host star such that it crosses our line of sight it can cause an apparent dip in the star’s brightness. These dips occur once every exoplanet year, the time it takes for the exoplanet to orbit around its host star. (Credit: NASA).

Many of these exoplanet transits were discovered using NASA’s Kepler and TESS missions. What these transits allow astronomers to do is measure the size of the exoplanet and its year. However, to verify that the object is an exoplanet, a series of spectra must be taken from which the mass of the object can be determined: for instance, the transiting object could be a low mass star or a brown dwarf (a failed star).

This diagram shows how an exoplanet can cause its host star to wobble around a common center of gravity which causes its light to be slightly blue and redshifted. Credit: ESO

NASA’s TESS mission is likely to continue for some years to come. However, TESS is best suited to identifying exoplanets orbiting low mass stars. Currently we know of no exoplanet which orbits a Sun-like star and has an orbit of around a year. Finding such exoplanets is the main goal of the European Space Agency’s (ESA) mission Plato. Its origin goes back 25 years ago when Eddington was proposed. Although this mission wasn’t finally selected by ESA, it led to the development of Plato which did get selected and funded and is due to be launched at the end of 2026.

Armagh’s Gavin Ramsay is one of the two ESA Community Scientist’s for Plato, whose role is to interact with the wider science community who are interested in using Plato data and is a member of the key ESA Plato Science Working Team which provides advice to ESA. The Science Working Team recently made a key decision: which patch of the sky should Plato first stare at? Since we want to detect Earth-like exoplanets orbiting a Sun-like star in the habitable zone, our stare needs to be at least for two years – this would detect two transits. The two key criteria were: is there enough Sun-like stars in the field and is there a ground-based spectrograph which could measure the motion of the transiting object around the host star?


The field selection was led by the group of Prof Giampaolo Piotto at the University of Padova in Italy. The spacecraft had to be able to observe the same part of the sky continuously for two years: this meant the potential fields had to be at relatively high ecliptic latitudes (away from the plane of the Solar System). Then there had to be enough Sun-like stars to give enough transits, but not close enough to the Milky Way that too many stars would lie within each pixel of the camera. The results from ESA’s Gaia satellite were central to this study. This resulted in one field in the north and one in the southern sky.


The next question: which spectrographs can reveal such low levels of motion? Jupiter causes the Sun to `wobble’ in its orbit by 12.8 m/s. However, the Earth causes the Sun to wobble by only 10 cm/s (a very slow walk!). Getting a spectrograph stable and sensitive enough is extremely difficult and this presentation gives great insight to what is required. The most sensitive spectrograph currently available is the European Southern Observatory (ESO) Espresso instrument which is attached to one of the VLT telescopes in Chile. The Science Working Team’s recommendation was therefore that the first Plato field would be in the Southern Sky.

The area of sky which will be observed first by Plato for at least two years. The different colours indicate how many cameras will observe the areas of the field, with the corners having six cameras and the center 24. (Credit: Valerio Nascimbeni/Giampaolo Piotto).

The Plato Science Working Team usually meets at ESA’s ESTEC laboratories in the Netherlands. At the last meeting we were given a treat by seeing the structural model in one of the labs where it was being put through various simulations for launch where the whole system would have to withstand the intense vibrations which would essentially shake the satellite and the telescopes. See this link for more details and photos!

A group of members of the ESA Plato Science Working Team at ESA’s ESTEC lab in the Netherlands, with the structural model of Plato with all 26 telescopes (not the final versions!) behind. The system was subject to vibrational tests to ensure the structure could withstand the vibrations which would be present during launch. Armagh’s Gavin Ramsay is in the front row on the right. (Credit: ESA)


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